1. Field of the Invention
This invention relates in general to well logging with a side entry sub, and in particular to a blowout preventer for sealing the sub in casing should a kick occur.
2. Description of the Prior Art
Most oil wells are logged at least once during drilling. In conventional logging, the drill pipe will be pulled, and one or more instruments are lowered on a conductor cable into the open hole to measure earth formation characteristics.
Deviated wells which may incline up to 65 degrees or more are common, particularly at offshore locations. A number of wells may be drilled from a single platform. These wells present difficult problems for logging. The logging instruments may not be able to reach bottom due to the inclination. Also, the cable dragging against the curved portion of the well may create a slot or key seat, causing the instrument to become stuck.
A recent technique has been developed to log deviated wells. In this technique, when it is desired to log a portion of the well, the drill pipe is pulled. The logging instruments are located in special housings and secured to the lower end of the drill pipe. The drill pipe is lowered into the well until it is located near the upper end of the zone to be logged.
Then, a side entry sub is secured to the upper end of the drill pipe. A side entry sub has a passage extending through its side wall for cable to pass. A latch is threaded through the side wall passage, and a packing is placed around the side wall passage. The latch is pumped down with drilling fluid pressure into electrical engagement with the instruments at the bottom of the well. The cable is laced under tension, and a clamp secures the cable to the side entry sub.
The string is then lowered farther into the well. Normally, tie wraps will be used to secure the cable to the exterior of the drill pipe as the string is lowered into the well. When the bottom of the well is reached, the side entry sub may be several thousand feet below the surface, but it will still be located in casing.
To log the well, the drill string is then pulled upward. The instruments are energized while each stand is pulled to log the well. Cable at the surface is simultaneously taken up. When the side entry sub again reaches the surface, the clamp is removed, and tension is applied to the cable to casue the latch to release from its connection to the logging instruments. The cable is then pulled from the drill pipe and the drill string is then removed normally.
Serious problems result if a kick occurs while the side entry sub is located in the well. "Kick" refers to pressure in the well forcing the drilling mud surrounding the pipe upwardly. A kick may occur when the hydrostatic pressure of the drilling mud is less than the formation pressure. If the kick is not controlled, the formation pressure will continue to blow the drilling mud from the hole, possibly resulting in an uncontrolled blowout and a fire. A blowout is a disaster, resulting normally in the loss of the well, the drilling rig, and huge sums spent in controlling the blowout.
During normal drilling, if such a kick occurs, blowout preventers located at the surface are actuated to close the annulus around the drill pipe. A blowout preventer has a large rubber element that is pushed inwardly to seal tightly around the drill pipe. Then, heavy mud is pumped down to overcome the pressure in the well.
If a kick occurs during side entry sub logging, however, the conventional blowout preventer will not seal around the drill pipe because the logging cable will be located on the exterior of the drill pipe. Pressure will leak through the contact point of the blowout preventer with the cable, causing erosion and leakage to continue. About all that can be done should a kick occur during side entry sub logging is to pull the string upward, cut the cable at a point as low as possible, then lower the string back into the well and close the blowout preventer. While this procedure allows the well to be controlled, it results in loss of the cable. Logging cable is expensive. Also, it will likely be difficult to pull the cable from the drill string subsequently.